There's a lovely Roland Barthes essay about the face you put on when you're photographed, to try to control how the image will come out. And it's the same when people look in the mirror. Like everyone, I put on a certain face, and I worry that what I see in the mirror is a construct. Also, my reflection is back to front, so it's not what anyone else sees.

I always get a shock when I look in the mirror because what I feel like on the inside doesn't correlate to what I look like on the outside. My mental image of myself changes daily. For instance, if I've got toothache, I feel like my face is totally swollen and that I am one great big tooth, but I look in the mirror and you wouldn't know. Or, if I've a nervous eye tic, I feel that I am a huge twitching eyeball, but I don't look like that in the mirror.

I've used mirrors in my art. I made a work called Your Authorised Reflection – a portrait-sized mirror that I signed back to front on the inside of the glass. The idea is that people look in it and their reflection is authorised or signed by me.

Then I made some clay busts of my own head and invited people to distort and change them. Somehow we ended up with something that felt close to a self-portrait.

It's strange: as an artist I work with notions of self-portraiture, but I find looking in the mirror a challenge. If I see a picture of myself in the newspaper, I don't like to look at it.